The present invention relates to a cryogenic heat exchange system in which a cryogenic heat transfer fluid is circulated through one or more passes of the heat exchanger in order to cool a heat load. Additionally, the present invention relates to a freeze dryer employing the cryogenic heat exchange system wherein the cryogenic heat transfer fluid is circulated through a condenser utilized in condensing sublimated water vapor.
Cryogenic heat exchangers are attractive design alternatives from the standpoint that they do not use environmentally damaging refrigerants, but instead use a cryogenic heat transfer fluid such as a liquefied atmospheric gas. Additionally, such cryogenic heat exchangers provide much greater flexibility in the amount of cooling provided and can reach colder temperatures than heat exchangers utilizing conventional refrigerants. It has been found, however, that it is difficult to build such a heat exchanger in a compact fashion because as the cryogenic heat transfer fluid enters the heat exchanger, more ice will build up on the side of the heat exchanger at which the cryogenic heat transfer fluid enters the heat exchanger. The section of the heat exchanger at which the ice has built up will be relatively ineffective as compared to the remainder of the heat exchanger. The ice itself may be unacceptable in come cases, such as in chilling liquids, or may block the heat exchanger. Still another problem is that there is very little control over the temperature of the heat exchanger. Assuming, liquid nitrogen were used as the cryogenic heat transfer fluid, the inlet to the heat exchanger would cool to temperatures of about 77 K. Such cooling would damage certain types of food products and in any event would be inefficient when the article to be cooled were only required to be cooled to about the freezing point of water.
As will be discussed, the present invention provides a cryogenic heat exchange system in which ice build-up on a heat exchanger employed in the cryogenic heat exchange system is more uniform (and possibly prevented altogether) as compared with that of prior art heat exchangers which utilize a cryogenic heat exchange fluid. Moreover, the present invention provides a cryogenic heat exchange system wherein the temperature at which heat transfer takes place can be controlled.